The ability of an endangered whale species to recover is jeopardized by increasing rates of entanglement in fishing gear and a resultant drop in birth rates, according to scientists who study the animal.

The population of North Atlantic right whales has slowly crept up from about 300 in 1992 to about 500 in 2010. But a study that appeared this month in the journal Frontiers in Marine Science said the number of baby right whales born every year has declined by nearly 40 percent since 2010.

Study author and Georgetown resident Scott Kraus, a scientist with the New England Aquarium in Boston who worked on the study, said the whales’ population suffers even when they survive entanglements in fishing gear. He said data suggest those “” on them.

“They are carrying heavy gear around, and they can’t move as fast or they can’t feed as effectively,” Kraus told The Associated Press in an interview. “And it looks like it affects their ability to reproduce because it means they can’t put on enough fat to have a baby.”

Entanglements have surpassed ship strikes as a leading danger to right whales in recent years. Forty-four percent of diagnosed right whale deaths were due to ship strikes and 35 percent were due to entanglements from 1970 to 2009, the study said. From 2010 to 2015, 15 percent of diagnosed deaths were due to ship strikes and 85 percent were due to entanglements, it said.

There is reason to believe the entanglements could harm conservation efforts despite recent positive signs on the whales’ recovery, Kraus said. Researchers said earlier this year that they were beginning to see more of the whales in Cape Cod Bay, and that was a good sign.

Right now, Atlantic right whales are in Canada waters off the Maritime Provinces and in the Gulf of Saint Lawrence.

“When entangled whales are spotted, they are not necessarily trailing gear from the year,” said the aquarium’s Tony Lacasse. “They can trail gear for more than year.”

“Most of these whales, unless caught in anchored gear, die when the ropes cut into the animal, and they die from systemic infection,” he said.

Charles “Stormy” Mayo, a senior scientist at the Center for Coastal Studies in Provincetown, said the drive to make fishing gear safer for the whales could be key to saving them.

“There’s a great deal of work being done to try to change the configurations of various kinds of fishing gear or the methods of fishing to reduce entanglement,” he told the AP.

NOAA Fisheries has implemented rule changes to reduce the risk of large whale entanglements associated with the buoy lines from trap and pot gear. That rule impacted all trap/pot fisheries along the Atlantic coast as well as a large closure area for all trap/pot fisheries in Cape Cod Bay and the outer Cape during February through April. Those rules went into effect in February 2015.

In May, Foggy the humpback whale was found entangled in fishing gear about 4 miles off the coast of Gloucester and Rockport. The 29-year-old whale was freed after a more than nine-hour rescue effort.

In January, the federal government added more than 18,000 nautical square miles of critical habitat of right whale feeding grounds off New England. The area now includes the entire Gulf of Maine, which stretches from Cape Cod to the southwest end of Nova Scotia, to southeast of Cape Cod.

North Atlantic right whales are among the most endangered species of whales. They spend the warm months feeding in areas off the Northeastern states and Canada and spend the winter off Southern states, where they give birth.

The leviathans may grow as large as 53 feet long and weigh as much as 70 tons. They may live to be 100 years old and tend to inhabit temperate waters in both the Atlantic and Pacific oceans.

There are estimated to be only 450 to 500 North Atlantic right whales left in the world, Kraus has said. Two of the 14 calves born last winter have died.

They are called right whales because they were hunted relentlessly during the whaling era, when they were considered the “right” whale to hunt because they were slow and floated when killed.

Mariners are urged to report any entanglement sightings of whales, sea-turtles and other marine animals to the Marine Animal Entanglement Response Hotline at 1-800-900-3622 or the US Coast Guard, and to stand by the animal at a safe distance until trained responders arrive.

Material from Patrick Whittle of the Associated Press was used in this report.